For example, a DC-DC converter is mounted on an automobile as a power supply device that supplies a DC voltage to various in-vehicle devices and circuits. Generally the DC-DC converter includes a voltage conversion circuit (boosting circuit or step-down circuit) that is constructed with a switching element and an inductor. The DC-DC converter outputs a DC voltage that is boosted or stepped down by switching the voltage of the DC power supply at high speed.
In some DC-DC converters, the voltage conversion circuit is activated only when the boosting or stepping-down is required, and power is supplied to a load through not the voltage conversion circuit but a bypass circuit when the boosting or stepping-down is not required. Switching elements such as a relay and an FET (Field Effect Transistor) are provided in the bypass circuit. Japanese Unexamined Patent Publication Nos. 2010-183755, 2010-174721, and 2013-74741 disclose power supply devices including the bypass circuits.
Japanese Unexamined Patent Publication No. 2010-183755 discloses a power supply device that performs control such that both the voltage conversion circuit and the bypass circuit are put into an operation state during switching between a first state in which the power is output through the voltage conversion circuit and a second state in which the power is output through the bypass circuit.
Japanese Unexamined Patent Publication No. 2010-174721 discloses a start-up control device including an operation monitor that monitors a voltage at an output terminal of the bypass circuit and a controller that drives a boosting circuit to maintain the voltage at the output terminal of the bypass circuit at a predetermined level.
In a power supply circuit disclosed in Japanese Unexamined Patent Publication No. 2013-74741, a rectifying switching element is provided instead of a rectifying diode, and the rectifying switching element is driven to supply the voltage to the load while a bypass relay is maintained in an off state when an engine is restarted.
In Japanese Unexamined Patent Publication No. 2013-74741, a thermistor is disposed near the rectifying switching element, and an abnormality of the rectifying switching element is detected when a voltage of the thermistor is greater than or equal to a threshold by temperature raise of the rectifying switching element. Japanese Unexamined Patent Publication Nos. 9-327120 and 2003-107123 also propose that temperature sensors such as the thermistor are provided near an element to detect the abnormality of the element. In Japanese Unexamined Patent Publication Nos. 9-327120 and 2003-107123, the temperature sensor detects a breakdown of a diode.
In the DC-DC converter in which the FET is used as the switching element of the bypass circuit, the FET is put into an on state to form a current path from the DC power supply to the load through the FET during non-conversion of the voltage. During the non-conversion of the voltage, if the controller outputs a false signal to put the FET into the on state, and a current continues to pass through a parasitic diode connected in parallel to the FET, the FET generates heat to raise the temperature. As a result, the FET degrades, and the FET burns out at the worst.